where the function u has the same periodicity as the lattice:
(where T is a lattice translation vector.)
A solution of this form can be plugged into the Schrödinger equation, resulting in the central equation:
and Ck and UG are the Fourier coefficients of the wavefunction ψ(r) and the potential energy U(r), respectively:
The vectors G are the reciprocal lattice vectors, and the discrete values of k are determined by the boundary conditions of the lattice under consideration.
In any perturbation analysis, one must consider the base case to which the perturbation is applied. Here, the base case is with U(x) = 0, and therefore all the Fourier coefficients of the potential are also zero. In this case the central equation reduces to the form
This identity means that for each k, one of the two following cases must hold:
If the values of are non-degenerate, then the second case occurs for only one value of k, while for the rest, the Fourier expansion coefficient must be zero. In this non-degenerate case, the standard free electron gas result is retrieved:
In the degenerate case, however, there will be a set of lattice vectors k1, ..., km with λ1 = ... = λm. When the energy is equal to this value of λ, there will be m independent plane wave solutions of which any linear combination is also a solution:
Non-degenerate and degenerate perturbation theory can be applied in these two cases to solve for the Fourier coefficients Ck of the wavefunction (correct to first order in U) and the energy eigenvalue (correct to second order in U). An important result of this derivation is that there is no first-order shift in the energy ε in the case of no degeneracy, while there is in the case of near-degeneracy, implying that the latter case is more important in this analysis. Particularly, at the Brillouin zone boundary (or, equivalently, at any point on a Bragg plane), one finds a two-fold energy degeneracy that results in a shift in energy given by:
This energy gap between Brillouin zones is known as the band gap, with a magnitude of .
In this model, the assumption is made that the interaction between the conduction electrons and the ion cores can be modeled through the use of a "weak" perturbing potential. This may seem like a severe approximation, for the Coulomb attraction between these two particles of opposite charge can be quite significant at short distances. It can be partially justified, however, by noting two important properties of the quantum mechanical system: